The following discussion is not an admission that anything discussed below is common general knowledge or background knowledge of a person skilled in the art.
U.S. Pat. No. 5,472,607 to Mahendran et al. describes a hollow fiber membrane comprising a braided tubular support coated on its outer surface with an asymmetric semipermeable film of polymer. Voids in the braided support are small enough to inhibit substantial penetration of a membrane forming dope. The polymer film extends over less than 33% of the outer portion of the braid's cross-sectional area. In one example, a tubular braid of glass fibers was coated with a dope of polyvinylidene-difluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). The braid had an inner diameter of 1.0 mm and an outer diameter of 1.5 mm. The complete microfiltration (MF) membrane had an outside diameter of 1.58 mm.
U.S. Pat. No. 6,354,444 to Mahendran et al. describes microfiltration (MF) or ultrafiltration (UF) membranes supported on a tubular braid. Various physical characteristics of a preferred braid are described. The braid may be made, for example, from polyesters or nylons. In a comparison test, membranes made of a PVDF in NMP dope coated onto a polyester braid were found to break less frequently than membranes with a glass fiber braid when used in an aerated, immersed microfiltration module.
Membranes generally as described in U.S. Pat. No. 6,354,444 are used in ZeeWeed membrane modules sold by GE Water and Process Technologies. These modules, originally developed by Zenon Environmental Inc., are possibly the most successful immersed membrane products for use in membrane bioreactors (MBR) ever made. However, although the membranes do not break, these membranes still occasionally suffer failures that occur when the membrane coating peels off of the braided support in harsh operating conditions. These delamination failures occur particularly at high stress points where the membranes enter the resin potting blocks. Although the fiber does nor break, delamination causes a breach in the rejection capability of the membrane.
In U.S. Pat. No. 7,807,221, Shinada et al. attempt to provide increased adhesion between a membrane material and a supporting braid by applying the membrane material in two separate coating layers. In U.S. Pat. No. 7,909,177, Lee et al. attempt to increase peeling strength by using fine filaments in the braid and adding stabilizing agents to the membrane dope to avoid forming large macro-void pores near the membrane to braid interface.